I have twin engine 496HO and have 2 Blue Top Optima's batteries paralleled together and while running 30+ mins at WOT, I get alarms indicating low voltage. What could be my issue?

a bad alternator and looking at the wiring it seems like everything is combined into one big system

https://www.manualslib.com/manual/14...Outlaw-35.html

I did find that I had one alternator bad, so I replaced it and now both alternators are pumping out 13.9 volts. But I am still getting voltage alarms. Could it be my blue tops? I thought they were the best out there.

http://pdfstream.manualsonline.com/d...a5d7392082.pdf

You will have to check the wiring to see that each motor is separate from each other
Remove the pos connection from one battery bank and see what works and what dosent , Write it down .ign, gauges, starting, etc.
Then connect it and remove the other pos lead, see what works and what dosent.write it down.

what are the capacity specs on the blue top batteries? You have to have a min of 800 cca in each battery per mercury. Have the batteries tested to see if they are any good. I would also recommend going away from blue tops